Field of the Invention
The present invention relates to an ophthalmic optical article which blocks light reflected off water, snow, glass and other surfaces to reduce glare by a polarizing function, and more particularly to a polarization optical article in which uneven coloring by polarization color has been improved, which is used as polarization sunglasses, polarizing correcting glasses, polarizing goggles, polarizing shields and the like.
Description of the Related Art
The basis of polarizing performances of a polarization optical article lies in a polarizer. Ordinary polarizer is a uniaxially stretched resin sheet doped with iodine or a dichroic dye (JP-B 2,663,440).
In the polarizer, an absorption axis and a transmission axis are defined with respect to a vibration direction of a light wave. Usually, a stretching direction of a uniaxially stretched resin sheet corresponds to an absorption axis, and the direction perpendicular to the absorption axis, namely the direction perpendicular to the stretching direction corresponds to a transmission axis.
The light wave of sunlight or a fluorescent lamp vibrates in any direction, the full 360 degrees. When this light wave is applied to a polarizer, a light wave component in an absorption axis direction is absorbed to the polarizer to cause a selective phenomenon in which the light wave component in a transmission axis direction penetrates. As a result, the light wave penetrated the polarizer is unified to only the component in a transmission axis direction. The polarizer having the polarization degree of 100% means that the entire (100%) light wave component in an absorption axis direction is absorbed and the entire (100%) light wave component in a transmission axis direction penetrates.
In addition to the uniaxially stretched resin sheet, a polarizer of a coating film molecular-oriented in a uniaxial direction also exists. However, the polarization principle is the same, and also physical brittleness is common because of a thin film. A description will be made below by way of a uniaxially stretched resin sheet as a typical example of a polarizer.
The uniaxially stretched resin sheet to be used as the polarizer is a thin film having a thickness of about several tens of μm and is very brittle, and thus it is difficult to use the polarizer alone as a polarization optical article, for example, sunglasses. Therefore, physical strength is increased by reinforcement through coating or laminating at least one side of the polarizer with the other resin layer. This reinforcing resin portion is called a protective sheet or a back-up resin portion.
The back-up resin portion is produced by injection molding (JP-A 8-52817) or cast molding (JP-A 2008-281791). However, there arose a new problem such as uneven coloring by polarization color caused by the back-up resin portion.
In order to make the description easier to understand, a description will be mainly made by way of the case of a polarizing lens.
In case of observing the polarizing lens in a state where another polarizer is laid on a back-up resin side of the polarizing lens thereby setting to a crossed nicols state, a phenomenon of separation into red, green and blue colors is observed in most cases. In the present invention, this color separation phenomenon is referred to as uneven coloring by polarization color. This uneven coloring by polarization color is called a polarization color due to birefringence.
Here are two kinds of reasons to consider, for uneven coloring by polarization color (hereinafter referred to as “uneven polarization coloring”), from the viewpoint of origin. One of them is uneven polarization coloring based on local molecular orientation formed in case of injection molding or extrusion molding of a back-up resin portion. This is referred to as uneven polarization coloring due to orientation birefringence.
The local molecular orientation appears at various positions of a plastic solid while varying the generation amount, size and direction depending on the molding conditions (molding temperature, molding rate, mold temperature, etc.), mold conditions (gate position, shape of mold, etc.) and the like. As a result, a local difference arises in orientation birefringence, which is called optical strain due to orientation birefringence, thus detecting as uneven polarization coloring.
The optical strain and uneven polarization coloring orientation due to orientation birefringence also appears in the polarizing lens, and an ordinary lens other than the polarizing lens in case of injection molding or extrusion molding of polycarbonate, PMMA, and a thermoplastic resin such as transparent nylon (JP-A 2002-187931).
However, the polarizing lens and ordinary lens differ in the observation method. Namely, in case of the polarizing lens, one polarizer is laid on a back-up side thereby setting to a crossed nicols state. In contrast, in case of the ordinary lens, the lens is interposed between two polarizers thereby setting to a crossed nicols state. If optical strain due to orientation birefringence exists, it can be detected as uneven polarization coloring.
In order to reduce uneven polarization coloring due to orientation birefringence, it is necessary to device a way based on molding conditions. Any method is a method of mitigating local molecular orientation caused by molding, and a decrease in injection molding rate and extrusion molding rate, a decrease in cooling rate of a mold and compression injection molding are required, and thus it is impossible to avoid drastic decrease in productivity, leading to an increase in costs.
There exists the technology in which already existing uneven polarization coloring due to orientation birefringence is reduced by orientational relaxation through an annealing treatment. However, heat deformation of the lens may occur concurrently with orientational relaxation in a thermoplastic resin.
In case of cast molding of a back-up resin portion, molecular orientation is less likely to occur during molding as a feature of a cast molding method. Accordingly, orientation birefringence is less likely to occur in the back-up resin portion, and thus optical strain and uneven polarization coloring due to orientation birefringence are less likely to occur.
In addition to uneven polarization coloring due to orientation birefringence, another kind of uneven polarization coloring exists. It is called uneven polarization coloring caused by optical strain due to photoelastic birefringence (hereinafter referred to as uneven polarization coloring due to photoelastic birefringence).
Uneven polarization coloring due to photoelastic birefringence is neither optical strain nor uneven polarization coloring, which originally exists in the back-up resin portion. For example, when the polarizing lens is mounted in a frame, the polarizing lens undergoes microdeformation due to a pressure of the frame which fastens the lens. The above-mentioned uneven polarization coloring due to photoelastic birefringence is acquired optical strain or uneven polarization coloring associated with the microdeformation. The method for observation of uneven polarization coloring due to photoelastic birefringence is the same as the method for observation of uneven polarization coloring due to orientation birefringence.
Optical strain due to orientation birefringence is optical strain involved in orientation properties (degree of orientation, direction, strain) of a main chain of resin molecules. In contrast, optical strain due to photoelastic birefringence is said to be optical strain involved in strain of a side chain of resin molecule. For example, in case of mounting in a frame, optical strain can be reduced or removed when a fastening pressure of the frame is decreased or the pressure applied to the lens are made uniform.
Optical strain and uneven polarization coloring due to photoelastic birefringence are generated regardless of the kind of the back-up resin and the molding method. Namely, even if the back-up resin portion is an extrusion-molded resin, an injection-molded resin, or a cast-molded resin, they are generated in any resin.
Well, is such optical strain due to orientation birefringence and photoelastic birefringence harmful to persons who wear eyeglasses?
Very interestingly, the injection-molded correcting lens made of polycarbonate, namely filled with optical strain due to orientation birefringence are now spread in the United States (a share will probably account for 50% or more of the whole correcting lens in the United States). If such optical strain due to orientation birefringence is harmful to the human eyes, the propagation of the correcting lens made of polycarbonate could not be achieved in the United States.
In the world, most of the correcting lenses made of plastic are oppupyed by cast-molded lenses (typical examples include CR39 lens). Upon observation of the cast-molded lens mounted in a glass frame in a crossed nicols state, uneven polarization coloring due to photoelastic optical strain is observed in most of lenses.
Taking the propagation of the correcting lens made of polycarbonate in the United States and grovel propagation of the cast-molded correcting lens into consideration, both optical strain due to orientation birefringence and optical strain due to photoelastic birefringence are considered to be substantially harmless to the human eyes.
Even if such optical strain due to birefringence is harmless, optical strain due to birefringence can be easily detected as uneven polarization coloring by setting two polarizers into a crossed nicols state.